Calibration of distance measuring equipment is generally done in a laboratory by comparison to a standard. Since field conditions are usually different from laboratory conditions, measurements may be in error because of the effects of the field conditions on the equipment. Often, correction factors may be applied, but just as often, there are uncorrectable variations in measurements. Moreover, equipment drifts with time and requires periodic recalibration to maintain its accuracy. Equipment is occasionally subjected to conditions outside the calibrated range but within the physical capacity of the equipment to resist damage. It would, therefore, be advantageous to perform calibration in-situ and without comparison to a standard for correction of drift and confirmation of calibration after over-ranging events.
Another aspect of laboratory calibration techniques is data reduction. Data collected in one set of units, for example electrical signal units such as volts or amps, must be multiplied by physical constants in another set of units, for example gravity or density, to obtain the final result. It would be advantageous to reduce the number of unit conversions required to reduce data collected for calibration and measurements.
Distance measurements are performed to determine fluid level using instruments including but not limited to, optical level detectors, electric resistance level detectors, sonic level detectors, radiofrequency level detectors, and pressure transducers. Distance measurements are made during surveying, for example with a theodolite instument, and while navigating, for example, with a sextant instument.
Improved accuracy of distance measurements is increasing in importance. For example, new requirements in environmental monitoring require measuring water depth in a well within 0.01 ft. Other applications that would benefit from highly accurate determinations of fluid level include but are not limited to gasoline storage tanks, food oil storage tanks, and chemical process tanks.
Establishing accuracy requires making many measurements in order to increase statistical confidence. By reducing the number of unit conversions as well as the number of physical operations necessary to obtain measurements, the requisite number of measurements for a particular confidence level may be made at a reduced cost.
Of the prior methods and apparati for measuring liquid level, none was found to provide in-situ calibration of the equipment. In the case where fluid level is determined using pressure transducers, it was found that all pressure transducers are subject to drift which requires correction by frequent calibration. Since calibrations are generally done at conditions different from the conditions under which measurements are obtained, this introduces a component of error or uncertainty that is difficult and expensive to overcome.
It would be advantageous to have a method and apparatus of in-situ calibration of distance measuring equipment to reduce the uncertainty of measurements without comparison to a standard, and to reduce the cost of obtaining the measurements. In particular, for measuring fluid level in wells, it would be advantageous to have a method and apparatus of in-situ calibration that would fit in most water wells and tanks and not require removal of the equipment from a well or comparison to a standard.